The present invention relates to a wettable solid cathode having an aluminide of at least one transition metal from groups IV B, V B and VI B of the periodic system and intended for use in a fused salt electrolytic cell to produce aluminum.
The production of aluminum by electrolysis of aluminum oxide involves dissolving the latter in a fluoride melt which is made up for the greater part of cryolite. The aluminum, which precipitates out at the cathode, collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum itself forming the cathode. Suspended from the overhead anode beam and dipping into the melt are anodes which in conventional processes are made of amorphous carbon. Oxygen is formed at the carbon anodes as a result of the electrolytic decomposition of the aluminum oxide. This oxygen combines with the carbon of the anodes to form CO.sub.2 and CO.
The electrolytic process takes place in general in the temperature range of about 940.degree.-970.degree. C. During the course of the process, the electrolyte becomes deplete in aluminum oxide. At a lower concentration of about 1 to 2 wt. % of aluminum oxide in the electrolyte the anode effect occurs whereby there is an increase in voltage from e.g. 4-4.5 V to 30 V and higher. Then at the latest the concentration of aluminum oxide in the melt must be raised by adding further aluminum oxide (alumina).
The use of solid, wettable cathodes in the electrolytic production of aluminum is already known. Suggested cathode materials are e.g. titanium diboride, titanium carbide, pyrolytic graphite, boron carbide and further substances including mixtures which can for example be sintered together.
Using wettable cathodes the normal interpolar distance can be reduced from about 5 cm to such a level as is permitted by other parameters such as the circulation of the electrolyte in the interpolar gap and the maintenance of the bath temperature. The smaller interpolar distance results in a significant reduction in energy consumption and also prevents the creation of irregularities in the thickness of the liquid aluminum layer.
In contrast to the wettable cathodes anchored firmly in the carbon floor of the cell the U.S. Pat. No. 4,243,502 reveals solid cathodes made of individually exchangeable elements each having at least one electrical current supply. In a further development according to the U.S. patent application Ser. No. 262,049, now U.S. Pat. No. 4,376,690, the exchangeable elements are made of two different parts which are rigidly connected by mechanical means and are resistant to thermal shock viz., an upper part projecting from the molten electrolyte into the precipitated aluminum, and a lower part situated only in the liquid aluminum. The upper part is made, at least in the region of the surface, exclusively of a material which is wet by aluminum, whereas the lower part or its coating is made of an insulating material which can withstand liquid aluminum.
The object of the U.S. patent application Ser. No. 317,189 now U.S. Pat. No. 4,410,412, is an exchangeable solid cathode which is made of an aluminide of at least one of the metals of the group of elements comprising titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, without employing metallic aluminum as a binder. The non-aluminum components of the aluminide belong therefore to group III B, IV B and/or VI B of the periodic table of elements.
The ability of the aluminides to withstand chemical and thermal effects permits them to be employed both in the molten electrolyte and in the molten aluminum, even though they exhibit limited solubility in the latter. This solubility, however, diminishes rapidly with decreasing temperature.
At the operating temperature of the reduction cell which is around 900.degree.-1000.degree. C. the solubility in liquid aluminum of a metallic component of the aluminide other than aluminum is approximately 1%. This means that the non-aluminum elements in the cathode are leached from it until the precipitated liquid aluminum is saturated with one or more of the transition metals in the aluminide.
The elements from the aluminides leached out during the reduction process are recovered from the precipitated metal by cooling this to about 700.degree. C. The aluminide crystallizing out of the liquid metal can be recovered by conventional means, and can be employed again in the production of cathode elements. The result is a recirculation of material with relatively little loss.
It is an object of the present invention to develop a solid cathode based on aluminides and with a service life equal to the lifetime of one or more anodes and such that the production and handling costs for the said cathode are substantially lowered.